ci: nightly-next tick runs on finished feat-cmux-next push runs, not a cron
User: no GitHub cron on the nightly path; nightlies as soon as possible. Replaces the 09:23/21:23 UTC schedule in nightly-next-tick.yml with workflow_run (completed) of cmux-next and cmux-tui artifacts push runs on feat-cmux-next, so a push whose own request was cancelled or stalled is picked up within minutes. Concurrency keeps at most one waiting tick. Drops the last reads of the retired Blacksmith variable CI_NIGHTLY_DECIDE_RUNNER.
Two commits (same as feat-cmux-next d8e0eecb98c, 191376a10f0): failing test first, then the fix. Dedicated builder: cmux22, plans/cmux-next/nightly-builder-DESIGN.md on feat-cmux-next.
![View with [code]smith](https://pr-comments-assets.blacksmith.sh/codesmith/view-with-codesmith-dark-v3.svg)
![Autofix with [code]smith](https://pr-comments-assets.blacksmith.sh/codesmith/autofix-with-codesmith-dark.svg)
Need help on this PR? Tag @codesmith-bot with what you need.
Summary by cubic
Makes nightly-next builds start as soon as possible instead of waiting for a scheduled cron.
Replaces the twice-daily schedule in nightly-next-tick.yml with a workflow_run trigger that fires whenever a cmux-next or cmux-tui artifacts push run on feat-cmux-next completes, whatever its conclusion. A push whose own request was cancelled or stalled is now picked up within minutes instead of hours; the concurrency group keeps at most one tick running and one waiting. Also drops the last reads of the retired CI_NIGHTLY_DECIDE_RUNNER variable.